The first answer to the question is that defrosting coils will not solve the problem .It sounds like you have a defrosting problem which can be one of three things, that,s if it,s not and electronic fridge.First it could be the timer or defrost thermostat[bi-metal] or the heater If you know where to find the timer,you can manually set in into defrost by turning the center button of the timer slowly clockwise until the fridge shuts off this is when it will be into defrost. Put you ear into the freezer section and listen for cracking and hissing noises after a few minutes. If you here this ,then your timer is bad.BUT if you here nothing then it could be you bi-matel or your heater, To check these parts you could jump the bi-metal when the freezer is cold inside,or check with a meter both bi-metal and heater to see if there is continity. BUT IF YOU JUMP THE BI-METAL AND FIND THE HEATER TO WORK DO NOT LEAVE IT JUMPED. REPLACE THE BI-METAL WITH A NEW ONE , BECAUSE THE BI-METAL IS A SAFTY DEVICE... GOOD LUCK